25th International Meeting on Organic Geochemistry IMOG 2011

P-336
Mechanistic model of the thermal evolution of n- and iso-alkanes
(C1-C32) in oils
Valérie Burkle-Vitzthum 1 , Roda Bounaceur 1 , Paul-Marie Marquaire 1 , François Montel 2 ,
Luc Fusetti 2
1 Laboratory of Reactions and Process Engineering, Nancy, France, 2 TOTAL Exploration and Production,
Pau, France (corresponding author:valerie.vitzthum@ensic.inpl-nancy.fr)
The purpose of the present work is to construct a
model able to take into account the cracking of a
complete distribution of n-alkanes as well as of a
distribution of branched alkanes from C1 to C32. The
choice of the iso-alkane model compounds is done on
the basis of the observations of biological systems
and sedimentary organic matter. The model takes into
account 46 iso-alkanes and 32 n-alkanes; this mixture
is meant to represent the major part of the saturated
fraction of petroleum.
A rigorous lumping together procedure is applied to
build the mechanistic model that consists of 13 206
lumped free radical reactions (initiations,
decompositions via �-scission, H-transfers, additions,
terminations) using rate constants available from the
literature. With this model, the cracking global
activation energy of n-C15 as well as iso-C15 is close
to 69 kcal/mol in the range 200-350 °C.
The model is validated on the basis of several
literature experimental results concerning pure alkane
cracking. The conversion as well as the product
distribution are well simulated up to 50% conversion
(300-450 °C and 90-700 bar).
The model is compared also to the saturated fraction
obtained from pyrolysis of Elgin oil (North Sea, UK) at
372 °C up to 1000 h. In this oil, the polar compounds
are negligible and the saturated fraction particularly
abundant. Fig. 1 shows the experimental product
distribution of the Elgin saturated fraction after whole
oil isochoric pyrolysis (TOTAL, unpublished data).
The C1-C10 fraction was analyzed by GCMS and the
C11+ fraction by GPC. The simulated alkanes
distribution is in Fig. 2. Gas and C10+ are well
represented whereas some discrepancies are
observed for C6-C10, but they are essentially due to
cycloalkanes that are not included into the model but
not negligible for these C numbers. In order to
characterize the C10+ decrease, the Ci+1/Ci ratios are
calculated: the model fits the experimental values up
to 500 h.
% mole
% mole
10
1
0,1
0,01
0,001
0,0001
0 10 20 30 40 50
Carbon number
Initial
30 hours
100 hours
200 hours
500 hours
1000 hours
Fig. 1: Elgin oil pyrolysis: experimental distribution.
100
10
1
0,1
0,01
0,001
0,0001
0,00001
0 5 10 15 20 25 30 35
Carbon number
initial
30 hours
100 hours
200 hours
500 hours
1000 hours
Fig. 2: Elgin saturated fraction: simulated distribution.
This model should lead to new insights into
thermogenic gas generation and relative thermal
reactivity of n- and iso-alkanes at a geological scale.
One additional potential application of our model is
the theoretical reconstruction of the real distribution of
n-alkanes of the produced oil from its analytical
distribution of iso-alkanes. Indeed, as opposed to isoalkanes,
the distribution of n-alkanes obtained from
the analysis of the produced oil is biased due to
partial precipitation of the n-alkanes population in the
subsea tubing.
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